Choosing your mate is an important decision. If you are a species that cares for your offspring, you and your mate need to be able to coordinate your care, and you must be able to rely on your mate to pull their weight. (Watch the albatross cam to see how the mated albatrosses depend on each other—one stays back with the chick while the other flies off to get food. If the food-getter never came back, or the chick-minder wandered off, the chick would die.) More fundamentally, whether or not you care for your offspring, you and your mate must be able to have biologically healthy offspring together.
Animals do not always get this right. The recently-in-the-news stories of Thomas the goose, who bonded with a black swan and spent his days helping that swan and his mate raise their cygnets, and Nigel the gannet, who was devoted to a concrete gannet dummy, are good examples of the errors an animal heart can make. From an evolutionary perspective, these are bad decisions: they prevent the lovestruck individual from passing on their genes to future generations. (All reports suggest that Thomas and Nigel appeared happy, so from an individual perspective, the mistake may not be so bad.)
But sometimes—very, very rarely—an error in mate choice, instead of being an evolutionary dead end, is the beginning of an entirely new lineage.